Oxide products of uranium have various utilities including preferred utilities as fuels for nuclear reactors in the nuclear industry and catalysts.
The performance of the fuel elements, traditionally enriched uranium dioxide structures clad in a metal container, is crucial to the practical success of the nuclear reactor. Nuclear power generation has imposed severe requirements on the performance of fuel in nuclear reactors, especially on properties of grain size and density of the fuel. It has been demonstrated that fine grain uranium dioxide structures are more subject tp creep than large grain uranium dioxide structures. It has also been discovered that the density of the uranium dioxide is a very important physical property influencing the performance of the fuel. In fabricated forms, uranium dioxide is a ceramic capable of compaction to give a structure of desired density and a low impurity level.
The enrichment of uranium customarily takes place through use of the compound uranium hexafluoride so that a process is required for convertiing the enriched uranium hexafluoride into enriched uranium dioxide in a form which can be readily fabricated to structures having a low fluoride content and a desired density and grain size.
Current practice for converting uranium hexafluoride to an oxide product of uranium, usually uranium dioxide, employs hydrolysis of uranium hexafluoride to give a solution of uranyl fluoride and hydrogen fluoride from which ammonium diuranate is precipitated by the addition of ammonia. After filtration the ammonium diurante of high fluoride content is dissolved in nitric acid with fluoride decontamination of the resulting uranyl nitrate solution being accomplished by solvent extraction. From the resulting purified uranyl nitrate solution, ammonium diuranate is reprecipitated and then calcined to give U.sub.3 O.sub.8 which in turn is reduced with hydrogen to give uranium dioxide.
Attempts have been made to replace this involved, expensive ammonium diuranate conversion process by gas phase reaction of uranium hexafluoride with a very successful method being described in copending U.S. Pat. No. 3,796,672 entitled Process for Producing Uranium Dioxide Rich Compositions from Uranium Hexafluoride which is hereby incorporated by reference. The foregoing application was filed Oct. 2, 1970 in the names of W. R. DeHollander and A. G. Dada and assigned to the same assignee as the present invention.
The practice of the process of U.S. Pat. No. 3,796,672 gives a uranium dioxide rich composition having particularly desirable properties and a gaseous atmosphere rich in reducing gas such as hydrogen. Since it is known that certain gaseous mixtures of a reducing gas such as hydrogen and air can e readily combustible and potentially explosive, it has been found desirable to convert any such gaseous mixture to its oxidized form during this process. Further a process sequence having a by-product gaseous atmosphere rich in a reducing gas such as hydrogen makes it undesirable to practice the process under vacuum condition because any air leaks in the process apparatus could result in localized explosive mixtures of hydrogen and air. Still further it would be desirable if this process could be improved to achieve uranium oxide compositions having higher oxide content such as U.sub.3 O.sub.8 (uranium tritaoctoxide) and still retain the desirable properties of the uranium dioxide rich powder produced in the process described and claimed in the foregoing patent.
In an attempt to complement the practice of the process of U.S. Pat. No. 3,796,672 and convert the residual reducing gas to its oxidized form, a new process was conceived for the conversion of uranium hexafluoride to a uranium oxide rich composition. This process is claimed in U.S. Pat. No. 3,790,493 and has the introduction of an oxygen-containing gas at a time when the uranium hexafluoride conversion to a uranium dioxide rich composition is substantially complete in the reaction zone. This achieves improvements in the flame conversion of uranium hexafluoride to an oxide product. Any reducing gas in the reaction zone, usually in the form of hydrogen, reacts to form its oxidized product and the uranium dioxide rich composition is converted to a higher oxide of uranium (hereinafter uranium oxide rich composition) with the particular oxide of uranium depending on the molar ratio of oxygen to the sum of the moles uranium dioxde rich composition and the residual reducing gas. This molar ratio can be changed by varying the volume of oxygen-containing gas introduced. This process permits a safe practice of the uranium hexafluoride conversionn under vacuum conditions. This process requires no separate heating step as the temperature of the intermediate reaction products of the uranium dioxide rich composition and residual reducing gas in the reaction zone is sufficient to react the residual reducing gas with the oxygen-containing gas downstream from the position at which the latter gas is introduced. This is very desirable since raising the temperature at this position in the reaction zone can lead to a partial sintering of the particles of the resulting uranium oxide rich composition. Since fine size particles of oxide are desirable, especially for catalytic applications, the partial sintering is usually undesirable.
This process will now be described in greater detail with reference to FIGS. 1 and 2 where there is shown a preferred embodiment of the invention having a reactor in which the above-described process of U.S. Pat. No. 3,790,493 can be carried out. This embodiment has two concentric tubes 33 aand 38 and the nozzle 30 is mounted and sealed by seals 37 in a supporting means such as a cover 31 which forms an air tight seal (which can be disconnected) with reactor vessel 32 defining a reaction zone 29. Vessel 32 has outwardly protruding space 34 which holds a pilot burner 35 which receives gas and maintains a pilot flame 36 to initiate a flame reaction.
The nozzle 30 has a first inlet means in the form of tube 33 with tubular inlets 47 for introduction of a fluid reactant and a second inlet means in the form of a tube 38 for introduction of another fluid. Tube 38 has inlets 39 and a cover 40 with an opening for a tubular inlet 41 for introduction of a fluid. A third inlet means is disposed in tube 38 in the form of a tubular chamber 43 defining a volume 42 for receiving fluid from inlet 41. Chamber 43 has eight openings in the portion 44 of size equal to the external diameter of tubes 45 which are connected to chamber 43 such as by welding or threading so that tubes 45 receive the fluid from chamber 43. Tubes 33 and 38 extend further into the reaction zone 29 than tubes 45 by the distance generally designated "d". A directional control plate 46 is secured transversely in the lower portion of tube 38 at a distance "l" above the open ends of tubes 45 and this plate 46 is provided with openings through which tubes 45 extend. The plate 46 coaxially forms an annular opening around each tube 45. Plate 46 forces the shielding fluid to pass through the annular openings and then into the reaction zone surrounding the jets of fluid reactant from tubes 45. The relation between the size of the holes in the plate 46 and the thickness of the plate 46 is such that the shielding fluid passes between the plate 46 and the tubes 45 in approximately unidirectional flow toward the reaction zone 29. The symbol "l" is used to designate the distance between the open ends of tubes 45 and the directional control plate 46.
In use, a ccontinuous flow of a reactant of a reducing gas selected from the group consisting of hydrogen, dissociated ammonia and mixtures thereof is maintained in tube 33 to the reaction zone 29 throughout the reaction so that there is a strong reducing atmosphere generally maintained in the reaction zone 29. A shielding gas is fed through inlets 39 into tube 38 and into the reaction zone 29. The shielding gas can be a non-reactive gas selected from the group consisting of nitrogen, argon, helium, neon, krypton, xenon and mixtures thereof or the shielding gas can be a reactive gas selected from the group consisting of oxygen, air, or a mixture thereof, or either air, oxygen or a mixture of air and oxygen with any of the foregoing non-reactive gases. A reactant comprising a mixture of uranium hexafluoride and an oxygen-containing gas is fed through inlet 41, chamber 42 and tubes 45 into reaction zone 29. The oxygen-containing gas is selected from the group consisting of oxygen, air and mixtures thereof. The flows of the gases in tubes 38 and 45 occur so that the shielding gas in tube 38 surrounds the jets of gaseous reactant coming from tubes 45 as the gases enter the reaction zone 29. The shielding gas shields the mixture of uranium hexafluoride and the oxygen-containing gas from the reducing gas for sufficient time so that the boundary of initiation of the reaction flame 48 in the reaction zone 29 is removed from contact with tubes 38 and this is referred to as a "lifted flame".
The oxygen-containing gas as the third reactant is introduced into reaction zone 29 through tubular members 50 so that the third reactant mixes with the reaction products of the primary flame 48. This results in formation of a secondary flame 51 due to the burning of the residual reducing gas to form its oxidized product and the conversion of the uranium dioxide rich composition to a composition rich in uranium oxides as previously described. The tubular members 50 are mounted so that the incoming third reactant gas enters the reaction zone 29 at the point where the uranium hexafluoride conversion to the transient particulate uranium dioxide rich composition is substantially complete. This patent in the names of Abdul G. Dada, W. R. DeHollander and Robert J. Sloat is assigned to the same assignee as the present invention and is hereby incorporated by reference.
Another very successful method of replacing the ammonium diuranate conversion process by gas phase reaction of uranium hexafluoride is described in copending U.S. patent application Ser. No. 387,529 entitled Process for Producing Uranium Oxide Rich Composition from Uranium Hexafluoride which is hereby incorporated by reference. This patent application was filed Aug. 10, 1973 in the names of W. R. DeHollander and C. P. Fenimore and is assigned to the same assignee as the present invention. This process gives the conversion of gaseous uranium hexafluoride to a uranium oxide rich composition in the presence of an active flame in a reactor defining a reaction zone by separately introducing a first gaseous reactant comprising a mixture of uranium hexafluoride and a reducing carrier gas and a second gaseous reactant comprising an oxygen-containing gas, the reactants being separated by a shielding gas as introduced to the reaction zone. The shielding gas temporarily separates the gaseous reactants and temporarily prevents substantial mixing and reacting of the gaseous reactants. The flame occurring in the reaction zone is maintained away from contact with the inlet introducing the mixture to the reaction zone. This process can also include a post oxidation step.
This post oxidation process for producing uranium oxide rich compositions has introduction of all the third gaseous reactant of an oxygen-containing gas at one location which gives a region of concentrated flame reaction and elevated flame temperature. The temperature of this post oxidation process is controlled to avoid loss of desirable ceramic properties of the resulting powder such as surface area. Further, the control of process temperatures can serve as a limitation on the production rate of the conversion reaction of uranium hexafluoride to a uranium oxide rich composition.